This application is directed generally to circuits and systems for communicating analog and digital signals and, in particular, to electronic circuits and systems for generating and communicating digital television signals.
For many years commercial television signals have been broadcast using a format known as NTSC. With the advent of high definition television and the increasingly occupied radio spectrum, it has been determined that television broadcasters will provide a simulcast of both NTSC signals and Digital television (xe2x80x9cDTVxe2x80x9d) signals so that the large installed base of NTSC receivers and the newly installed DTV receivers will be simultaneously able to enjoy the same broadcasts. Thus, simultaneous broadcasting will involve the simultaneous transmission of identical programs encoded in two different formats over respective television channels. Both the NTSC and the DTV channels have been defined as being 6 MHZ in bandwidth.
Simultaneous broadcasting in both NTSC and DTV formats will involve the use of two different channels. Because the available spectrum for the additional channels is quite limited, it has been proposed that the additional channels be allocated in channels which are nearby to other existing channels. Where both NTSC and DTV signals are being transmitted over nearby channels, there exists a considerable problem of co-channel interference where the NTSC and DTV signals interfere with each other. The interference problem between nearby channels will be worsened if signals carried on the DTV and/or NTSC waveforms are not precisely located and are not kept within their assigned channel bandwidth.
In proposed DTV systems, and as shown in FIG. 1, a signal bearing video and audio data may be sent from a signal source, Such as a television studio to a RF/Transmission site which may not be co-located therewith. In such a system, the video and audio data signals may be transmitted (or transported) from the studio to the RF/Transmission site using conventional communications techniques, such as microwave links. This signal, often referred to as the Transport Signal, will contain both the data and the clock for the data, either separately or by the use of a self-clocking coding such as Manchester coding. Conventionally, the RF/Transmission facility would use the data clock for the purpose of recovering the data at the Transmission facility. Thus, if the Transfer layer clock is inaccurate or drifts, the effect of the inaccuracies would work their way into the signal which is eventually broadcast from the Transmission facility. Given the importance of maintaining accurate broadcast frequency, the frequency errors caused by inaccurate clocks at the Transport layer (often the studio) may be unacceptably high.
As described below, in one embodiment of the present invention, the inaccuracies in the data clock can be removed or compensated by the system utilized at the RF/Transmission site so that the signal which is broadcast has stable frequency components.
As described below with respect to another embodiment of the present invention, the problems of co-channel and cross channel interference may be based, at least in part, in differences between the clocks used to generate or transmit the video and/or audio signals from their generation to their broadcast. Such interference may be reduced or eliminated by the use of a common clock to serve as the base for different portions of the signal generation and broadcast Systems described herein.
Accordingly, it is an object of the present invention to provide a novel circuit and system for generating a digital television signal in which the frequency based components are precisely generated and/or maintained.
It is another object of the present invention to provide a novel circuit and system for compensating and/or adjusting for inaccuracies in the data clock prior to broadcasting of the television signal.
It is yet another object of the present invention to provide a novel circuit and system that can operate on two asynchronous clocks without synchronous handshaking.
It is further an object of the present invention to provide a novel circuit and system for adding a pilot signal to a television signal.
It is still another object of the present invention to provide a novel circuit and system for error handling in a television signal.
It is yet still an object of the present invention to provide a novel circuit and system for television transmissions where the data signal has its dependency on the frequency of the digital data rate removed and the frequency corrected to its nominal condition.
It is further still an object of the present invention to provide novel circuits and systems for trellis encoding and novel multi-band antennas for a television system.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.